Radiant heater having formed filaments



Oct. 22, 1963 Az vARY 3,108,171-

RADIANT HEATER HAVING FORMED FILAMENTS Filed June 18'. 1962 INVENTORALEX VARY BY jflu, ATTORNEYS United States Patent 3,108,171 RADIANTHEATER HAVING FORMED FILAMENTS Alex Vary, North @lmsted, Ohio, assignorto the United States of America as represented by the Administrator ofthe National Aeronautics and Space Administration Filed June 18, 1962,Ser. No. 203,411 5 Claims. (Cl. 219-49) (Granted under Title 35, U.S.Code (1952), see. 266) The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

The present invention is concerned with a high-temperature heatingdevice, and more particularly, with a refractory metal radiant heatersuitable for operation in a vacuum. The heater of the present inventionuses moderate currents and voltages from an alternating current sourceor a direct current source.

High temperatures in the range between 2200 K. and 2800 K. have beenattained over surfaces of several hundred square centimeters usingeither resistance or high frequency induction heating. Radiant heatingwith refractory metal resistance elements, such as molybdenum ortungsten wire or ribbon, has been proposed for use in small scalehigh-vacuum applications such as thermionic devices. To increase thescale to the kilowatt range, however, it is necessary to bring theresistive elements into intimate contact with refractory supports.Because of the resulting gaseous products at high temperatures, thistype of heating has not been satisfactory for many vacuum applications.

Some resistance heaters are constructed of refractory metal tubesappropriately slotted to lengthen the electrical path. Such heaters havevery low electrical impedance and, therefore, require as much as severalthousand amperes to operate at the kilowatt level. Also, the highcurrent demand of resistance and induction heaters requires bulkywater-cooled leads and expensive power supplies.

To eliminate the need for cumbersome equipment and to facilitate theconstruction of appropriate vacuum enclosures, it has been proposed touse high impedance radiant heaters which have applications in a widerange of activity from metallurgical to thermionic studies. With theimposition of the criteria that such devices be selfsupporting in thehot zone and that they have small cross-sectioned elements, certainstructural problems are posed where large areas are to be heated.Because in many research areas it is desirable to operate heaters withdirect current to avoid alternating current pickup in objects beingstudied, high impedance devices are needed to reduce current demand atkilowatt inputs to values below one hundred amperes.

It is, therefore, an object of the present invention to provide animproved high-temperature heater element for electronic devices andVacuum furnaces which has a low current demand together with a minimumamount of distortion and temperature sag.

Another object of the invention is to provide an improved radiant heaterfor electronic devices, such as plasma diode thermionic converters,which may be useful in a variety of high temperature vacuum deviceshaving a large radiation area.

A further object of the invention is to provide a hightemperatureheating device which cancels its induced magnetic held and eliminatesheavy leads, supports, and accompanying cooling requirements.

A still further object of the invention is to provide a self-supportinghigh temperature heater in which all ce- 3, 1 9-8, 1 7 l Patented Get.22, 1963 ramic and other high vapor pressure components are maintainedoutside the hot zone.

Other objects and advantages of the invention will become apparent fromthe specification which follows and from the drawing in which likenumerals are used throughout to identify like parts.

In the drawing:

*FIG. 1 is a perspective view with parts broken away of a heating deviceconstructed in accordance with the invention;

FIG. 2 is an enlarged perspective view showing the detailed constructionof a typical filament utilized in the heating device illustrated in FIG.1; and

FIG. 3 is an enlarged sectional view taken along the line 3--3 in FIG.1.

In order to achieve the aforementioned objects, there is provided ahigh-temperature heating device (for use in a vacuum which has aplurality of refractory metal strips for forming parallel filaments thatare arranged in a generally circular array. Each of the filaments iscurved about its major centerline to a dihedral angle for structuralstability, and the strips are connected to form a series circuit byconnecting opposite ends of each of the strips to alternately adjacentstrips to effect magnetic flux cancellation as an electric current ispassed through the circuit.

Referring now to the drawing, there is shown in FIG. 1 a heating device10 constructed in accordance with the present invention which isutilized to heat a thermionic device, such as tubular cathode 12 shownin FIG. 3. This thermionic device may also be an anode. The heatingdevice It) utilizes a plurality of formed filaments 14 of a refractorymetal, such as tantalum, tungsten, or molybdenum. As best seen in FIGS.2 and 3, each of the formed filaments 14 comprises an elongated stripthat is transversely curved about its major centerline to a dihedralangle so that each filament has a pair of angularly disposed surfaces 16and 18 extending longitudinally along opposite sides of the centerline.Very good results have been obtained when the angle X is approximately60 because the heat emitted from the surfaces 16' and 18 is directedoutward toward the cathode 12 and the filaments 14 are self-supportingin the hottest portions of the heater 10.

The filaments 14 are arranged in a generally circular array of a pair ofannular spacing members in the form of rings 20 and 22 of a ceramicmaterial, such as magnesium silicate or alumina. As best seen in FIG. 3,the vert-ices of the dihedral angles may be considered to representregularly spaced elements on an imaginary cylinder containing theheater. This is accomplished by crimping the extreme lower portion 24 ofeach filament 14 as shown in FIG. 2 about a bottom support rod 26 whichprotrudes upwardly from a blind hole 28 in the ring 22 as shown in FIG.1 and crimping the extreme upper portion 30 about a top support rod 32that is received in a through hole 34 in the ring 20. The holes 23 and34 are slightly larger than the support rods 26 and 3-2, respectively,to provide optimum clearance for unrestricted expansion and contractionof the support rods. The heater 10 can expand uniformly as a unit, andin the process the upper ring 20 slides loosely along a tube 36 which itencircles. An important feature of the invention is that both of therings 20 and 22 which support the filaments 14 are located outside thecentral hot zone of the heater 10.

The tube 36 extends along the central portion of the circular array offilaments 14 and through the rings 20 and 22 as shown in FIG. 1. Thetube 36 is of an electriaclly conducting refractory metal, such astantalum, tungsten, or molybdenum, and this tube serves not only as aheat reflector, but also as an electrical lead.

A hollow cylinder 38 which is likewise of an electrically conductingrefractory metal has an outer diameter substantially smaller than theinner diameter or the tube 36 as shown in FIG. 3, and the cylinder 38extends through the tube 36. The elongated cylinder 38 is maintained outof electrical contact with the tubular conductor 36 by a pair of ceramicinsulators 4d and 42 as shown in FIG. 1. The elongated cylinder 38 formsa basic support for the heater 10' and is rigidly secured to acantilever support 44 with an insulating member 46. The cantileversupport 44 is rigidly mounted in a con ventional manner in the system,and the mounting of this member forms no part of the present invention.

Power is supplied to the tubular conductors 36 and 3 8 from leads 48 and50, respectively, which extend through a base 52 for the heater 1%). Thelead 48 is connected to the tubular conductor 36 by an electricallyconducting bracket 54 that is secured to this conductor immediatelybelow the ring 22 as shown in FIG. 1. Likewise, a similar bracket 56 isconnected to the mating cylindrical conductor 38 immediately below theinsulator 42.

The tubular conductor 36 is placed in electrical contact with one of theformed filaments 14 by an electrically conducting bar 53 as shown inFIGS. 1 and 3. Likewise, an adjacent filament v14 is placed inelectrical communication with the inner cylindrical conductor 38 by asimilar bar 60 which extends through a slot 62 in the tubular conductor36.

A series circuit is formed between the bars 58 and oil by electricallyconnecting opposite ends of each of the filaments 14 to alternatelyadjacent filaments with connectors 64 so that the current in adjacentfilaments is flov ing in opposite directions -to efieot a magnetic fluxcancellation. More particularly, the current flows from the lead 48through the bracket 54 to the tubular conductor 36, whereupon it flowsto the filament 14a through the bar 58. The current flows upward throughthe filament 14a to the connector 64a, then downward through thefilament 14b to the connector 64b, and upward through the filament 140to connector 64c. The circuit continues in this manner until the currentflows through connector '64q to the filament 141' and then to thecylindrical conductor 38 through the bar 60, where it passes to the lead50 through the bracket 56. A feature of the invention is that tight fitsbetween the above-mentioned parts are not required because imperfectelectrical contact is immediately remedied by self-welding caused by hotspots arising upon the application of the current.

A typical heater 10 having a length of six inches and a diameter of oneinch was constructed in accordance with the invention. Each filament 14was formed from a strip of tantalum four inches long, 1 inches Wide, andten mills thick, and this strip was bent about its major centerline to adihedral angle of sixty degrees. At the strip ends, /2 inch segments 24and 30 were made tubular to provide a snug fit over forty mill diametersupport rods 26 and 32. The conductors 36 and 38 consisted of twocoaxial tubes having a thickness of ten mills. The outer tube 36 had a/2 inch outside diameter and the inside tube 38 had a A inch outsidediameter.

This heater was operated under an average pressure of l0- mm. Hg atvarious temperatures above 2000 K. on a current of fifty-six amperesfrom a twenty kva. autotransformer. It is contemplated that aseventy-five volt generator would be adequate for a direct currentoperation. The flux density was measured at a Vs inch radial distancefrom the aforementioned imaginary cylinder and was found to be two gaussat approximately fifty amperes as compared to a reference of seventeengauss along a straight Wire.

While the preferred embodiment of the invention has been illustrated anddescribed, it is contemplated that various structural modifications maybe made without departing from the spirit of the invention or the scopeof the suojoined claims. For example, an alternate mode of operation iscontemplated which consists of applying a sufficiently high potential tothe member 12 that is being heated so as to induce electron bombardment.

What is claimed is:

1. A high temperature heating device comprising a plurality oftransversely curved strips of refractory metal for forming elongatedfilaments to conduct an electric current,

a pair of annular members for supporting said strips parallel to oneanother in a generally circular array,

a first spaced supports carried by one of said annular members,

second spaced supports carried by the other of said annular members,

means for securing one of said first supports to each of said strips atone end thereof,

means for securing one of said second supports to the opposite end ofsaid strip,

a tubular conductor extending along the central portion of said circulararray through said annular members to form a heat reflector,

an elongated conductor extending through said tubular conductor,

means for insulating said elongated conductor from said tubularconductor,

means for supporting said elongated conductor in a substantially rigidposition,

means for supplying electric power to said conductors,

and means for connecting said strips to said conductors in series.

2. A high temperature heating device as in claim 1, including spacedceramic annular members having holes for receiving said first and secondspaced supports, each of said holes having an inside diameter greaterthan the outside diameter of its corresponding support.

3. A high temperature heating device as in claim 2, including spacedmagnesium silicate members for receiving said supports, said membersbeing located on opposite sides of a centrally disposed hot zone in saidheating device.

4. A high temperature heating device as in claim 2, including spacedalumina members for receiving said supports, said members being locatedon opposite sides of a centrally disposed hot zone in said heatingdevice.

5. A high temperature heating device as in claim 1, wherein one of saidannular members is located above the other annular member, and saidcurved strips extend vertically between said members.

References Cited in the file of this patent UNITED STATES PATENTS1,954,203 Gurtler Apr. 10, 1934 2,057,931 Stupakofi' Oct. 20, 19362,553,875 Shaw May 22, 1951 2,596,327 Cox et a1. May 13, 1952 2,662,990Kohl Dec. 15, 1953 2,727,177 Bailey et a1. Dec. 13, 1955 2,747,074'Finch May 22, 1956 2,748,310 Agule May 29, 1956-

1. A HIGH TEMPERATURE HEATING DEVICE COMPRISING A PLURALITY OFTRANSVERSELY CURVED STRIPS OF REFRACTORY METAL FOR FORMING ELONGATEDFILAMENTS TO CONDUCT AN ELECTRIC CURRENT, A PAIR OF ANNULAR MEMBERS FORSUPPORTING SAID STRIPS PARALLEL TO ONE ANOTHER IN A GENERALLY CIRCULARARRAY, A FIRST SPACED SUPPORTS CARRIED BY ONE OF SAID ANNULAR MEMBERS,SECOND SPACED SUPPORTS CARRIED BY THE OTHER OF SAID ANNULAR MEMBERS,MEANS FOR SECURING ONE OF SAID FIRST SUPPORTS TO EACH OF SAID STRIPS ATONE END THEREOF, MEANS FOR SECURING ONE OF SAID SECOND SUPPORTS TO THEOPPOSITE END OF SAID STRIP, A TUBULAR CONDUCTOR EXTENDING ALONG THECENTRAL PORTION OF SAID CIRCULAR ARRAY THROUGH SAID ANNULAR MEMBERS TOFORM A HEAT REFLECTOR, AN ELONGATED CONDUCTOR EXTENDING THROUGH SAIDTUBULAR CONDUCTOR, MEANS FOR INSULATING SAID ELONGATED CONDUCTOR FROMSAID TUBULAR CONDUCTOR, MEANS FOR SUPPORTING SAID ELONGATED CONDUCTOR INA SUBSTANTIALLY RIGID POSITION, MEANS FOR SUPPLYING ELECTRIC POWER TOSAID CONDUCTORS, AND MEANS FOR CONNECTING SAID STRIPS OT SAID CONDUCTORSIN SERIES.